This invention relates to acoustic attenuators and relates more particularly to an improved attenuator for reducing acoustic noise in gas turbine engines.
The present invention is concerned with the attenuation of noise generated by gas turbine engines, in contrast to various prior art attenuators, mufflers or the like, that are concerned with reduction in jet noise generated in the atmosphere by the exhaust flow from a gas turbine engine. Various theoretical studies have been conducted to determine the different sources of noise from the gas turbine engine itself. This noise is relatively broad band in nature extending throughout the audible frequency range. It is believed that boundary layer flow is a source of low frequency noise, while at least prominent high frequency noise peaks appear related to the turbine blade passing frequency. Various harmonics of these and other noise sources result in a relatively broad band acoustic noise spectrum.
Many prior art attempts to reduce gas turbine engine noise have centered about introduction of cooling airflow into the hot gas exhaust flow from the engine to reduce and/or better homogenize exhaust flow temperature once its thrust and/or work has been accomplished. In many instances however, it is impractical or too costly to make provisions for introduction of cooling air flow into the hot exhaust gas. Many other prior art attempts have centered about deswirling the exhaust gas flow to reduce noise sources. Generally these prior art arrangements increase back pressure on the exhaust gas flow to reduce overall engine efficiency, and/or alter the pattern of exhaust flow which in many instances may be aerodynamically undesirable. For instance, previous attempts to reduce engine noise by minimizing thermal gradients in the exhaust flow characteristically impose significant back pressure penalties. Many times these prior art structures are also characterized by relatively bulky, expensive, heavy and complicated muffler absorption devices.